Pharmacological basis for the induction of gastric carcinoid tumours in the rat by loxtidine, an insurmountable histamine H2-receptor blocking drug.

The very late occurrence of gastric carcinoids in a life-span carcinogenicity study with loxtidine in the rat might have resulted from continuous achlorhydria induced by this long-acting unsurmountable histamine H2-antagonist. The nature of the anti-secretory activity of loxtidine was compared with that of ranitidine on histamine-induced acid secretion in the perfused stomach preparation of the rat and in the rat isolated gastric mucosa preparation. Ranitidine and loxtidine had qualitatively different inhibitory effects on acid secretion, ranitidine being a competitive antagonist of histamine even at high concentrations, whereas the effect of loxtidine on both preparations was unsurmountable at relatively low concentrations. These results support the hypothesis that the late formation of gastric carcinoids in rats receiving loxtidine is a consequence of persistent achlorhydria caused by unsurmountable blockade of parietal cell H2-receptors.